Chemical sensors which use a fluorescent indicator for measurement of concentration based on emitted fluorescent intensity as a main parameter, have suffered from drift and inaccuracy results caused by changes in source intensity or ambient temperature.
The following attempts to alleviate these disadvantages have been proposed but only with a partial success.
One suggestion was that the temperature has to be measured by a thermometer and that of the source intensity by a separate detector. The measured fluorescent intensity was corrected based on Tables which have a precalibrated data set which serve for temperature compensation. However, this compensation does not cover all factors which cause drift, such as: photobleaching, variation of the reagent properties after sterilization and variations of intensity caused by other optical components of the system except the emitting source.
Another proposal was to incorporate a second fluorescent reagent that emits light at a different wavelength and is not sensitive to the measured chemical changes. However, only a partial optical compensation could be achieved which does not include the differences in spectral response of detector.
A further suggestion, was to measure the lifetime of the excited state, instead of the intensity. In this case, there is not any compensation for the ambient temperature and therefor an additional thermometer is necessary. Also, a pulsed light source with time constants in the order of magnitude like the lifetime of the excited electronic state of the fluorescent reagent must be used. The bandwidth of the detector and electronics is large, so that a corresponding large intensity of light will be required. In this manner, the method for the manufacture of this sensor is quite complicated and costly.
Another suggestion was to use the fluorescent reagent immobilized in a rigid polymer, such as plexiglass, as a reference. However, this method does not provide any compensation for differences in environmental effects on fluorescent material and actually was never used, due to the absence of complete compensation.
Japanese Kokai No. 59-170748 describes a method for determining the presence of oxygen in an environment, which consists of exposing a sensor therein and measuring the quenching related by the decrease in intensity. A reference device with areas of differing size or concentration of luminiscent material is immobilized in a support, which preferably is a polymer, that is relatively impermeable to oxygen. The indicators used consist of luminiscent inorganic materials which luminisce when excited with visible or ultraviolet light and whose luminiscence is quenchable by oxygen. The luminiscent materials which are mentioned belong to platinum group metal complexes. The luminiscent reagent on the reference element is spread in a band ranging from low to high concentrations. The measurement itself is done by comparing visually the light intensity of the signal with the light intensities of the band on the reference sensor. The main disadvantage of this method is due to the subjective determination carried out by eye, which of course can not be accurate enough. As mentioned in the specification, the precision accuracy of oxygen determination is about 2%, which actually should be considered a semiquantitative oxygen determination. Of course in those cases where a very high precision is required, this method can not be used.
It is an object of the present invention to provide a method for an accurate determination of the concentration of gases, vapours or dissolved gases in a sample. It is another object of the present invention to provide a method for an accurate determination of the concentration of gases, vapours or gases dissolved in a sample, which compensates for the variation caused by the change in concentration of the respective gas. It is yet another object of the present invention to provide a method for an accurate determination of the concentration of gases, vapours or gases dissolved in a sample, wherein said determination is objective being measured automatically.